The present invention relates to the fabrication of cutting elements, particularly of the type in which a diamond layer is adhered to a carbide substrate to form a composite, and the composite is bonded to a support stud.
One type of cutting element used in rotary drilling operations in earth formations comprises an abrasive composite or compact mounted on a stud. The composite typically comprises a diamond layer adhered to a cemented carbide substrate, e.g., cemented tungsten carbide, containing a metal binder such as cobalt, and the substrate is brazed to the stud. Mounting of the cutting element in a drill bit is achieved by press-fitting or otherwise securing the stud into predrilled holes in the drill bit.
Fabrication of the composite is typically achieved by placing a cemented carbide substrate into the container of a press. A mixture of polycrystalline diamond grains and catalyst binder is placed atop the substrate and is compressed under ultra-high pressure and temperature conditions. In so doing, metal binder migrates from the substrate and "sweeps" through the diamond grains to promote a sintering of the diamond grains. As a result, the diamond grains become bonded to each other to form a diamond layer, and that diamond layer is bonded to the substrate along a planar interface. Metal binder remains disposed in the diamond layer within pores defined between the diamond grains.
A composite formed in that manner may be subject to a number of shortcomings. For example, the coefficients of thermal expansion of cemented carbide and diamond are close but not exactly the same. Thus, during heating or cooling of the composite, thermally induced stresses will occur at the interface between the diamond layer and cemented carbide substrate, the magnitude of the stresses being a function of the disparity in the thermal expansion coefficients.
Another potential shortcoming which should be considered relates to the creation of internal stresses within the diamond layer which can result in a fracturing of that layer. Such stresses can result from the presence of the metal binder within the diamond layer, since the metal binder possesses a much higher coefficient of thermal expansion than the diamond. Thus, the diamond and metal binder will expand or contract at different rates during temperature changes, whereby internal stresses are created in the diamond layer. If the metal binder is uniformly dispersed throughout the diamond layer, the stresses will be, in effect, self-balancing. However, if the metal binder is not uniformly distributed, localized (concentrated) stresses will be established which can lead to a fracturing of the diamond layer. A uniform dispersion of the metal binder would minimize that problem but is difficult to achieve.
The above shortcomings have been recognized in the prior art and efforts have been made to overcome them, as exemplified by the disclosure of European Patent Application No. 0133 386. In that disclosure, it is proposed to provide a polycrystalline diamond body which is completely free of metal binders and a carbide backing. Such a diamond body is to be set directly in a metal support, according to the disclosure in the European Patent Application. However, the mounting of a diamond body directly in metal presents significant problems as regards an inability of the metal to provide sufficient support for the diamond body. The European Patent Application proposes to deal with that problem by providing irregularities, such as spaced ribs in the bottom surface of the diamond body, which ribs are to be embedded in the metal support.
According to the European Patent Application, the irregularities can be formed in the diamond body after the diamond body has been formed, e.g., by laser or electronic discharge treatment, or during the formation of the diamond body in a press, e.g., by the use of a mold having irregularities. As regards the latter, it is suggested in the European Patent Application that a suitable mold could be formed of cemented carbide; in such a case, metal binder would migrate from the mold and into the diamond body, contrary to the stated goals in the European Patent Application of eliminating the carbide substrate and the presence of metal binder in the diamond layer. The European Patent Application proposes to deal with such an inconsistency by immersing the thus-formed diamond/carbide composite in an acid bath which would dissolve the carbide mold and leach all metal binder from the diamond body. There would thus result a diamond body containing no metal binder and which would be mounted directly in a metal support. Notwithstanding any advantages which may result from such a structure, significant disadvantages exist as explained below.
In sum, the European Patent Application proposes to eliminate the problems associated with the presence of a cemented carbide substrate and the presence of metal binder in the diamond layer by completely eliminating the cemented carbide substrate and the metal binder. However, even though the absence of metal binder renders the diamond layer more thermally stable, it also renders the diamond layer less impact resistant. That is, the diamond layer is more likely to be chipped by hard impacts, a characteristic which presents serious problems during the drilling of hard substances such as rock.
Furthermore, the absence of metal binder in the diamond layer eliminates a potential safeguard against fracturing of the diamond layer during so-called "back conversion". That is, it may occur during heating of the diamond layer, e.g., during the brazing of the composite to the stud, or during a cutting operation, that the heated diamond grains tend to convert back into graphite. This imposes stress on the diamond grains which can lead to the formation of cracks in the diamond layer. However, the presence of metal binder within the diamond layer resists the propogation of those cracks, due to the relative ductility of the metal binder as compared to the stiff diamond grains.
It will also be appreciated that the direct mounting of a diamond body in a metal support will not alleviate the afore-discussed problem involving the creation of stresses at the interface between the diamond and metal, which problem results from the very large difference in the coefficients of thermal expansion between diamond and metal. For example, the thermal expansion coefficient of diamond is about 45.times.10.sup.-7 cm./cm./.degree.C. as compared to a coefficient of 150-200.times.10.sup.-7 cm./cm./.degree.C. for steel. Thus, very substantial thermally induced stresses will occur at the interface. In addition, once the portions of the diamond which do not carry the ribs begin to wear sufficiently to expose the metal therebehind, that metal will wear rapidly, due to its relative ductility and lower abrasion/erosion resistance, to undermine the integrity of the bond between the diamond and the metal support.
Understandably, then, the elimination of both a cemented carbide substrate and the presence of metal binder from the diamond layer does not represent an ideal solution to the problems confronting the use of diamond cutting elements in drilling operations.
It is, therefore, an object of the present invention to minimize or obviate problems of the above-discussed type.
A further object is to increase the performance of cutting elements of the type employing diamond/cemented carbide composites.
An additional object is to provide a composite comprised of a cemented carbide substrate carrying a diamond layer, the diamond layer containing metal binder which is highly uniformly dispersed at least in the area of the interface between the diamond layer and the substrate.
A further object is to minimize the magnitude of thermally induced stresses occurring at the interface between the diamond and cemented carbide substrate.